U.S. patent number 7,019,908 [Application Number 10/909,167] was granted by the patent office on 2006-03-28 for modulator circuitry.
This patent grant is currently assigned to Mapper Lithography IP B.V.. Invention is credited to Ernst Habekotte, Floris Pepijn van der Wilt, Johannes Christiaan van 't Spijker, Marco Jan-Jaco Wieland.
United States Patent |
7,019,908 |
van 't Spijker , et
al. |
March 28, 2006 |
Modulator circuitry
Abstract
The invention relates to a modulator for modulating the
magnitude of a beamlet in a multi-beamlet lithography system, said
modulator comprising at least one means for influencing the
direction of a beamlet, a light sensitive element for receiving
light from a modulated light beam and converting said light into a
signal, and discretizing means, coupling to said light sensitive
element and to at least one of means for influencing, for
converting said signal received from said light sensitive element
into a discrete signal having discrete values selected from a set
of predefined discrete values and providing said discrete signal to
said means for influencing.
Inventors: |
van 't Spijker; Johannes
Christiaan (Delft, NL), Wieland; Marco Jan-Jaco
(Delft, NL), Habekotte; Ernst (Delft, NL),
van der Wilt; Floris Pepijn (Delft, NL) |
Assignee: |
Mapper Lithography IP B.V. (CJ
Delft, NL)
|
Family
ID: |
34103022 |
Appl.
No.: |
10/909,167 |
Filed: |
July 29, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050062950 A1 |
Mar 24, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60491475 |
Jul 30, 2003 |
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Current U.S.
Class: |
359/619; 359/618;
359/291 |
Current CPC
Class: |
H01J
37/3177 (20130101); B82Y 40/00 (20130101); B82Y
10/00 (20130101); G03F 7/70291 (20130101); G03F
7/70383 (20130101); H01J 2237/0435 (20130101) |
Current International
Class: |
G02B
27/10 (20060101); G02B 26/00 (20060101) |
Field of
Search: |
;359/298,245-251,618,619,291,855,224 ;385/17-19,24 ;708/816 ;355/67
;345/697 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Mack; Ricky L.
Assistant Examiner: Hasan; M.
Attorney, Agent or Firm: Blakely Sokoloff Taylor &
Zafman
Parent Case Text
This application claims the benefit of Provisional Application No.
60/491,475, filed Jul. 30, 2003.
Claims
The invention claimed is:
1. A modulator for modulating the magnitude of a beamlet in a
multi-beamlet lithography system, said modulator comprising: at
least one means for influencing the direction of a beamlet; a light
sensitive element for receiving light from a modulated light beam
and converting said light into a signal; discretizing means,
operationally coupling to said light sensitive element and to at
least one of means for influencing, for converting said signal
received from said light sensitive element into a discrete signal
having discrete values selected from a set of predefined discrete
values and providing said discrete signal to said means for
influencing.
2. The modulator according to claim 1, further comprising a passage
for said beamlet, wherein said means for influencing are located
near said passage.
3. The modulator according to claim 2, said modulator being a
modulator for modulating the magnitude of a charged particle
beamlet in said multi-beamlet lithography system, said means for
influencing comprising at least one electrode, located near said
passage for generating an electrical field for influencing said
charged particle beamlet, and said discretizing means coupling to
said light sensitive element and to at least one of said at least
one electrodes.
4. The modulator according to claim 1, said modulator being a
modulator for modulating the magnitude of an optical beamlet in a
multi-beamlet lithography system, said means for influencing being
located in the optical path of said beamlet.
5. The modulator according to claim 4, said means for influencing
comprising at least one deflector for deflecting an optical
beamlet.
6. The modulator according to claim 5, further comprising means for
modifying the deflection angle of the deflector, said means for
modifying being coupled to said discretizing means.
7. The modulator of claim 1, wherein said discretizing means
comprises at least one comparator circuit, which provides the
functionalities of: providing a first difference between said
signal and a reference signal; providing a second difference
between at least one threshold value and said first difference;
providing a discrete value selected from said set of predefined
discrete values, based on the magnitude of said second
difference.
8. The modulator of claim 1, wherein said discretizing means
comprises a discretizing circuit comprising: A first
differentiator, coupled to said light sensitive element, for
calculating a first differential signal from the signal and a
reference signal, A second differentiator, coupled to said first
differentiator, for calculating a second differential signal from
said first differential signal and a threshold signal; A converter,
coupled to said second differentiator for converting said second
differential signal into discrete values, selected from said set of
predefined discrete values.
9. The modulator according to claim 8, wherein the discretizing
circuit is an inverter circuit comprising a transistor of the
PMOS-type and a transistor of the NMOS-type.
10. The modulator according to claim 9, the discretizing circuit
comprising a number of inverters in series, wherein at least one
inverter is provided with a transistor of the PMOS-type and a
transistor of the NMOS-type.
11. The modulator according to claim 10, wherein said number of
inverters in series is an even number.
12. The modulator according to claim 11, wherein each consecutive
inverter in said number of inverters in series comprises
transistors designed to operate with a higher current than the
transistors in the previous inverter in said number of inverters in
series.
13. The modulator according to claim 1, wherein said signal is a
time-varying electrical potential.
14. The modulator according to claim 1, wherein said signal is a
time-varying electrical current.
15. The modulator according to claim 1, wherein the discretizing
means comprises thresholding means for defining said discrete
values.
16. The modulator according to claim 15, wherein said thresholding
means hold at least one predefined threshold value.
17. The modulator according to claim 15, wherein said thresholding
means is adapted fro defining a threshold value based on the
signal.
18. The modulator according to claim 15, wherein said thresholding
means is adapted for defining a threshold value based on a previous
state of the signal, and/or based on a differential of the signal,
and/or based on an integral of the signal and/or previous discrete
value.
19. A modulator array comprising a plurality of modulators, for
modulating a maganitude of a plurality of beamlets in a
multi-beamlet lithography system, each modulator comprising: at
least one means for influencing the direction of a beamlet; a light
sensitive element for receiving light from a modulated light beam
and converting said light into a signal; discretizing means,
operationally coupling to said light sensitive element and to at
least one of means for influencing, for converting said signal
received from said light sensitive element into a discrete signal
having discrete values selected from a set of predefined discrete
values and providing said discrete signal to said means for
influencing.
20. The modulator array according to claim 19, wherein the
discretizing means comprises thresholding means for defining said
discrete values.
21. The modulator array according to claim 20, wherein said
thresholding means hold at least one predefined threshold
value.
22. The modulator array according to claim 20, wherein said
thresholding means is adapted for defining a threshold value based
on the signal.
23. The modulator array according to claim 20, wherein said
thresholding means is adapted for defining a threshold value based
on a previous state of the signal, and/or based on a differential
of the signal, and/or based on in integral of the signal and/or
previous discrete value.
24. A multi-beamlet lithography system, comprising: a source for
generating a plurality of beamlets; a modulator array comprising a
plurality of modulators for modulating the magnitude of each
individual beamlet individually; pattern data generating means for
generating pattern data representing a pattern to be transferred by
the lithography system, and optical transmission means for
optically transmitting the pattern data to the modulator array in
at least one an optical signal, wherein said modulator array
comprises at least one optical receiver for receiving the at least
one optical signal, the receiving means comprising discretizing
means for converting the at least one optical signal into discrete
signals having discrete values selected from a set of predefined
discrete values and providing said discrete signals to said
modulators.
25. The lithography system of claim 24, wherein said discretizing
means are adapted for converting the at least one optical signal
into at least one discrete signal for substantially each
modulator.
26. A method for transferring a pattern onto the surface of a
target using a lithography system comprising beam generating means
for generating a plurality of beamlets and modulation means for
individually controllably modulating substantially each beamlet,
said method comprising: retrieving pattern data from data storage
means; transforming said pattern data into at least one modulated
light beam; optically coupling said at least one modulated light
beam to said modulation means comprising an array of modulators
with at least one modulator for each beamlet, each modulator
comprising: at least one means for influencing the direction of a
beamlet; a light sensitive element for receiving light from a
modulated light beam and converting said light into a signal;
discretizing means, operationally coupling to said light sensitive
element and to at least one of means for influencing, for
converting said signal received from said light sensitive element
into a discrete signal having discrete values selected from a set
of predefined discrete values and providing said discrete signal to
said means for influencing.
27. A modulator for modulating the magnitude of a beamlet in a
charged particle beamlet lithography system, said modulator
comprising: at least one means for influencing the direction of a
beamlet; a light sensitive element for receiving light from a
modulated light beam and converting said light into a signal;
discretizing means, operationally coupling to said light sensitive
element and to at least one of means for influencing, for
converting said signal received from said light sensitive element
into a discrete signal having discrete values selected from a set
of predefined discrete values and providing said discrete signal to
said means for influencing, said modulator being a modulator for
modulating the magnitude of a charged particle beamlet in said
charged particle lithography system, said means for influencing
comprising at least one electrode, located near said passage for
generating an electrical field for influencing said charged
particle beamlet, and said discretizing means coupling to said
light sensitive element and to at least one of said at least one
electrode.
Description
BACKGROUND
The invention pertains to a modulator for modulating the magnitude
of a beamlet in a multi-beamlet lithography system, in particular
for a maskless lithography system.
Maskless lithography systems, including ion, laser, EUV and
electron beam systems, all require means to process and deliver
pattern data to some kind of writing means. Since a mask is a
highly efficient way to store a pattern, the amount of raw data to
describe a pattern is enormous. Moreover, for a commercially
acceptable throughput, the data must be transported towards the
writing means at a very high data rate. Additionally the high data
rate must be obtained within limited space. It was up to now not
recognized that improvement of the data path in maskless
lithography systems has a profound effect on the throughput of
these systems.
All maskless lithography systems can be divided in two classes. In
the first class, the data are sent towards the individual radiation
source or sources. By tuning the intensity of the sources at the
right times, a pattern can be generated on the substrate, which is
most often a wafer. The switching of sources may get problematic
when the switching speed increases. The settling time of a source
can for instance be too long.
The second class of maskless lithography systems on the other hand
comprises either continuous sources or sources operating with a
constant frequency. The pattern data are now sent towards
modulation means, which completely or partly stop the emitted beams
from reaching the target exposure surface when necessary. By
controlling these modulation means while moving over the target
exposure surface, a pattern is written. The modulation means are
less critical for settling times. A lot of maskless lithography
systems designed to achieve a higher throughput are therefore using
modulation means.
Canon discloses in U.S. Pat. Nos. 5,834,783, 5,905,267 and
5,981,954 a maskless electron beam lithography system with one
electron source. The emitted electron beam is expanded, collimated
and additionally split by an aperture array into a plurality of
beamlets. A blanker array fed with pattern data stops the
individual beamlets when a control signal is given. The obtained
image is then reduced by a reduction electron optical system and
projected on a wafer.
Advantest on the other hand discloses a maskless electron beam
lithography system in patent applications US20010028042,
US20010028043, US20010028044, WO02054465, WO02058118 and
WO02058119, wherein a plurality of electron sources is used. The
emitted electron beamlets pass a blanker array, which deflects the
individual electron beamlets when the appropriate control signal is
given. Furthermore the passing electron beams are shaped by a
shaping array, and eventually focused on the wafer.
Micronic describes in for instance patent application WO0118606 and
U.S. Pat. No. 6,285,488 an optical lithography system that uses a
spatial light modulator (SLM) to put information into the emitted
light beam. A light source emits light pulses directed towards the
SLM. The SLM comprises an array of deformable mirrors, which
reflect the emitted beam towards a substrate or towards a beam stop
structure depending on the control signal sent to the mirror
involved.
The information on a mask is normally used to transfer a pattern
from the mask on a certain area on the target exposure surface.
This area is called a die. To get an idea of the amount of data
that has to be transferred, imagine a die of 32 mm by 26 mm. Now
consider that somebody wants to write a pattern with a critical
dimension (CD) of 45 nm. Then there are 4,1*10.sup.11 CD-elements
on a die. If each CD element consists of at least 30*30 pixels to
satisfy the requirements, and if there is only one bit needed to
represent the intensity of said pixel, the information present on a
mask is represented by about 3,7*10.sup.14 bits. Say a commercially
acceptable throughput for a maskless lithography system is about 10
wafers/hr. If there are 60 dies on a wafer, 60 times 3,7*10.sup.14
bits have to be transported towards the modulation means per wafer.
So 600 times 3,7*10.sup.14 bits have to be transported towards the
modulation means in 3600 seconds to get the desired throughput.
This corresponds to a data transfer rate of about 60 Tbit/s!
In all mentioned systems the control signals are sent
electronically towards the modulation means. However, the bandwidth
of a metal wire is limited. The limit on the bandwidth of an
electrical interconnect is related to the maximum total capacity of
an electrical interconnect B.sub.max, to the overall cross-section
A and the length of the electrical interconnect L in the following
way: B.sub.max=B.sub.0*(A/L.sup.2)
The constant of proportionality B.sub.0 is related to the
resistivity of copper interconnects. For typical multichip module
(MCM) technologies B.sub.0 is about 10.sup.15 bit/s. For on-chip
lines its value is about 10.sup.16 bit/s. The values are almost
independent on the particular fabrication technology.
The limit on the bandwidth of the electrical interconnect is
furthermore independent of its configuration. Whether the
interconnect is made up of many slow wires or a few fast wires up
to the point where other effect start to limit the performance
makes no difference.
The desired total capacity of the electrical interconnect is
100*10.sup.12=10.sup.14 bit/s. This corresponds to a ratio of the
overall cross-section to the square of the length of the electrical
interconnect of 10.sup.-1 in the case of a MCM and 10.sup.-2 in the
case of an on-chip connection. So if L is 1 m, the overall
cross-section of copper is 0,01 0,1 m.sup.2! Compare that number
with the size of a die that is written, which is 0,0008 m.sup.2,
and it is evidently impossible to establish the data transfer
without a demagnification of at least 10 after the pattern
information is added to the light beam.
Another approach to visualize the problem is to use the typical
speed of an electrical interconnect, which is in the order of 1
Gbit/s. So to transfer 100 Tbit/s, 100.000 copper wires are needed!
This takes an enormous amount of space and is difficult to
handle.
In U.S. provisional application 60/421,464, filed on Oct. 25, 2002,
incorporated by reference, as a solution to this problem it is
suggested to feed pattern data using optical transmission
means.
SUMMARY OF THE INVENTION
An objective of the current invention is to improve the systems
described above.
A further or other objective of the current invention is to provide
a modulator which enables stable operation of an optical or charged
particle beam lithography system.
A further other objective of the current invention is to provide a
modulator which improves the dose control.
The invention therefore provides a modulator for modulating the
magnitude of a beamlet in a multi-beamlet lithography system, said
modulator comprising: at least one means for influencing the
direction of a beamlet; a light sensitive element for receiving
light from a modulated light beam and converting said light into a
signal; discretizing means, coupling to said light sensitive
element and to at least one of means for influencing, for
converting said signal received from said light sensitive element
into a discrete signal having discrete values selected from a set
of predefined discrete values and providing said discrete signal to
said means for influencing.
Using optical data transportation means in a lithography system
makes it possible to create a maskless lithography system based on
known technology, but having an increased throughput and
stability.
Furthermore, the discretizing means improve the reliability of a
lithography system based on these modulators, as small fluctuation
in the signal will not lead to unintended displacements and
aberrations of the beamlets.
Furthermore, it provides a possiblity to control the dose more
accurately.
In the context of the invention, the word `light` is used in the
sense of optical (electromagnetical) radiation. In particular,
optical radiation in a wavelength range of about 200 to about 2000
nm is used.
For generating beamlets which transfer the actual pattern onto the
surface of a substrate, the radiation source that can be used can
emit any kind of radiation having a short wavelength, like
electrons, positrons, x-rays, photons or ions. The source is either
a continuous source or a source that is pulsed with a continuous
frequency. The source therefore does not generate any information,
but only generates beamlets. However, the purpose of a lithography
system is to pattern a certain target exposure surface. Since the
source does not provide any pattern information, the pattern
information has to be added to the beamlets somewhere along their
trajectory by modulation means. It should in this invention be
realized that the pattern information is transported using an
optical system. The pattern information is used to control
modulation means, which modulates beamlets, which actually write
the pattern into a resist or in another way transfer the pattern
onto a sample, for instance a semiconductor wafer. In the system,
the nature of the pattern writing beamlets depends on the nature of
the source. In fact, the modulated light beams are pattern
information carrying light beam, and the beamlets are
pattern-writing beamlets.
In an embodiment, the discretizing means comprises an electric
circuit, operationally coupled to the light sensitive element and
to the means for influencing the direction of a beamlet.
The modulation means can operate in different ways and be based on
various physical principles, depending on the nature of the
beamlets used for writing the pattern. It may generate a signal,
which results in the activation of some blocking mechanism, which
stops the beamlet, for instance a mechanical shutter or a crystal
becoming opaque due to electro-acoustic stimulation. Another
possibility is that the modulation means selectively generate a
signal, which results in the activation of some sort of deflector
element, like an electrostatic deflector or a mirror. This results
in a deflection of the selected irradiated beamlet. The deflected
beam is then projected on a blanker element, for instance a beam
absorbing plate provided with apertures, aligned with the
deflectors of mirrors. In both cases a commercially satisfactory
throughput can only be acquired, when the beamlet modulation is
done very fast, preferably with a frequency of 100 MHz or more.
In maskless lithography systems the pattern information is
represented by computer data, generally digital computer data. The
pattern data is partially or completely stored in the control unit.
The control unit therefore comprises a data storage medium, e.g.
RAM, hard disks or optical disk, for instance a plurality of these
media provided in a parallel configuration. This data is stored in
a format that can be used to control the modulation means in such a
way that a predetermined pattern can be repetitively generated.
Furthermore, the control unit comprises means to read out the data
at a high data rate. To establish the high data rate the control
unit comprises an element that converts the data into at least one
pattern data carrying light beam. In an embodiment, this data
converter comprises a vertical cavity surface emitting laser
(VCSEL) diode. If a bit is one, a light signal is emitted while no
light is sent out if the value of the bit equals zero, for
instance. By reading out a sequence of bits, a pattern information
carrying light beam is created. The pattern information carrying
light beams are then transported towards the modulation means.
There are several possible carriers that can realize the data
transfer.
In an embodiment the transfer from the converter element in the
control unit to a region close to the modulation means is achieved
using optical fibers for the data transport. This allows flexible
data transport with minimal disturbance by electromagnetic fields
and other means.
Currently the optical fibers that are for instance used in telecom
and Ethernet applications are optimized for specific wavelengths,
predominantly 850, 1300 and 1500 nm. The 850 nm optimization is
established due to the good availability of the standard
InGaAs/GaAs laser diodes. The infrared wavelengths are used because
of the low fiber transmission losses, typically smaller than 0.4
dB/km. Future developments aim for wavelengths of 660 and 780 nm.
The lower wavelengths are preferred for the present invention
because of fewer diffraction related limitations at these
wavelengths. However, in some configurations larger wavelengths are
desired. The wavelengths that can be used in the present invention
are optical wavelengths and range from about 200 to 1700 nm.
Current developments furthermore make it possible to transfer
multiple signals through one channel. For this purposes either
multi-wavelength or multimode optical fibers are developed, and
multiplexing/demultiplexing techniques are used.
In an embodiment of the invention, each modulator of the modulation
means comprises a light sensitive element for converting said at
least one modulated light beam coming from said control unit into a
signal for actuating said modulator. In a further embodiment
thereof, said optical transmission means comprise at least one
optical fiber for transmitting said at least one modulated light
beam from said control unit to said modulation means. In this
embodiment, said at least one optical fiber at its modulation means
end is coupled to one or more optical fiber arrays. In a further
embodiment thereof, substantially every optical fiber from said one
or more optical fiber arrays is coupled to one of said light
sensitive converter elements.
In an alternative embodiment, said at least one optical fiber at
its modulation means end is coupled to one or more optical
wave-guides, and said optical wave-guides being coupled to the
light sensitive elements.
In an embodiment of the maskless lithography system described
above, said optical transmission means comprise at least one
multiplexer at its control means end and at least one demultiplexer
at its modulation means end.
In another embodiment of the maskless lithography system described
above, it has an optical path parallel to which said plurality of
beamlets are traveling, wherein said optical transmission means are
furthermore provided with optical coupling means for coupling said
at least one modulated light beam into said optical path.
In embodiment described above, the data converter and the optical
transmission means are adapted for generating at least one
modulated light beam having at least one wavelength between 200 and
1700 nm.
In a further embodiment of the invention, each light sensitive
element is provided with a selection filter, which is transparent
for a predetermined wavelength range.
In an alternative embodiment, each light sensitive element is
provided with a selection filter for transmitting light having a
predetermined direction, or range of directions, of
polarization.
In another alternative embodiment, each light sensitive element
comprises a prism which limits the sensitivity of said light
sensitive element to light entering said prism from a predetermined
direction, of range of directions.
In yet another embodiment, each light sensitive element comprises a
grating, which limits the sensitivity of said light sensitive
element to light entering said grating from a predetermined
direction, or range of directions.
In a further embodiment of the maskless lithography system
comprising optical fibers, said light sensitive element comprises a
photodiode, in an embodiment a MSM-photodiode, a PIN-photodiode or
an avalanche photodiode.
In an embodiment of the maskless lithography system with an optical
fiber array, the modulator comprises an electrostatic
deflector.
In an embodiment of the maskless lithography system according to
the present invention, the data converter comprises a laser
diode.
In an embodiment wherein the source comprises an electron beam
generating means, each modulator of the modulation means comprises
a light sensitive element for converting said at least one
modulated light beam coming from said control unit into a signal
for actuating said modulator, said modulation means has a beam
generating means side and a target side, each of said modulators
comprises at least one electrostatic deflector, an aperture between
said at least one electrostatic deflector and said target side,
said electrostatic deflectors of said modulators make up an
electrostatic deflector array and said apertures of said modulators
make up an aperture array, each electrostatic deflector is coupled
to a light sensitive element, the light sensitive elements are
located at the beam generating side of said modulation means and
said electrostatic deflectors are located between said light
sensitive elements and said aperture array. In this embodiment,
said optical transmission means comprise beam splitting means for
splitting said at least one modulated light beam into a plurality
of modulated light beams, and optical couplers for coupling each
modulated light beam to a light sensitive element.
In this embodiment, said optical couplers comprise projecting means
for projecting said plurality of modulated light beams on said
light sensitive elements at an angle between 0 and 80 degrees
relative to a plane perpendicular to said electrostatic deflector
array. In this embodiment, the projecting means comprise at least
one lens for projecting the plurality of modulated light beams on
said electrostatic deflector aperture array.
In an embodiment of the electron beam maskless lithography system,
the projecting means comprise a first demagnifier with a reduction
optical system for demagnifying the plurality of modulated light
beams and a projection optical system for projecting the
demagnified plurality of modulated light beams on said
electrostatic deflector aperture array. In an embodiment thereof,
said reduction optical system comprises a micro lens array, each
micro lens of said micro lens array being aligned with one of said
plurality of modulated light beams and adapted for reducing the
size of said one of said modulated light beams. In a further
embodiment thereof, said projection optical system further
comprises a mirror, for reflecting the plurality of modulated,
demagnified light beams coming from the reduction optical system in
the direction of said lens of the projection optical system.
In an embodiment of the electron beam maskless lithography system
described above, the area on the modulation means not covered by
the light sensitive elements is provided with a reflective
layer.
In an embodiment of the electron beam maskless lithography system
described above, a diffusive layer is provided on the surface of
the modulation means facing the incoming plurality of modulated
light beams.
In an embodiment of the electron beam maskless lithography system
described above, said optical transmission means further comprises
an optical wave guide for coupling each of the plurality of
modulated light beams substantially parallel to the electrostatic
deflector aperture array plane through said array towards its
corresponding light sensitive element. In a further embodiment
thereof, the optical transmission means further comprises an
optical micro lens array provided with a plurality of micro lenses,
each micro lens being aligned with one of said plurality of
modulated light beams for coupling its modulated light beam into a
corresponding optical wave-guide.
In an embodiment of the electron beam maskless lithography system
described above, the optical transmission means comprising a
plurality of optical fibers, the data converter means comprising
means for operationally coupling said at least one modulated light
beam to or in said plurality of optical fibers, said plurality of
optical fibers being grouped to form at least one fiber ribbon,
said at least one fiber ribbon being attached at one of the sides
of said electrostatic deflection array, and the light sensitive
elements being adapted for electrically activating their
corresponding electrostatic deflector via electrical
interconnects.
In another embodiment, the maskless lithography system, the
generating means comprise light beam generating means. In an
embodiment thereof, the light generating means are adapted for
generating a light beam having a wavelength smaller than 300 nm. In
a further embodiment thereof, the modulation means comprises a
spatial light modulator. In a further embodiment thereof, the
spatial light modulator comprises a deformable mirror device,
comprising an array of micro-mirrors. In yet a further embodiment
thereof, each micro-mirror comprises a light sensitive element
mounted on its backside coupled to said optical transmission means
for receiving a modulated light beam.
The invention further relates to a process wherein a maskless
lithography system is used described above.
The invention further relates to a method for transferring a
pattern onto the surface of a target using a lithography system
comprising beam generating means for generating a plurality of
beamlets and modulation means for individually controllably
modulating substantially each beamlet, said method comprising:
retrieving pattern data from data storage means; transforming said
pattern data into at last one modulated light beam; optically
coupling said at least one modulated light beam to said modulation
means.
In an embodiment of this method the modulation means comprise an
array of modulators, each provided with light sensitive elements,
and method further comprises: splitting said at least one modulated
light beam into a plurality of modulated light beams; coupling each
of said modulated light beams to one light sensitive element.
In an embodiment, the modulator according to the invention further
comprises a passage for said beamlet, wherein said means for
influencing are located near said passage;
In a further embodiment thereof, this modulator is a modulator for
modulating the magnitude of a charged particle beamlet in a
multi-beamlet charged particle lithography system, said means for
influencing comprising at least one electrode, located near said
passage for generating an electrical field for influencing said
charged particle beamlet, and said discretizing means coupling to
said light sensitive element and to at least one of said at least
one electrodes.
In a further embodiment, the modulator according to the invention
is a modulator for modulating the magnitude of an optical beamlet
in a multi-beamlet lithography system, said means for influencing
being located in the optical path of said beamlet.
In an embodiment thereof, said means for influencing comprises at
least one deflector for deflecting an optical beamlet.
In a further embodiment thereof, the modulator further comprises
means for modifying the deflection angle of the deflector, said
means for modifying being coupled to said discretizing means.
In an embodiment of the modulator of the current invention, said
discretizing means comprises at least one comparator circuit, which
provides the functionalities of: providing a first difference
between said signal and a reference signal; providing a second
difference between at least one threshold value and said first
difference; providing a discrete value selected from said set of
predefined discrete values, based on the magnitude of said second
difference.
In an embodiment of the modulator of the current invention said
discretizing means comprises a discretizing circuit comprising: A
first differentiator, coupled to said light sensitive element, for
calculating a first differential signal from the signal and a
reference signal, A second differentiator, coupled to said first
differentiator, for calculating a second differential signal from
said first differential signal and a threshold signal; A converter,
coupled to said second differentiator for converting said second
differential signal into discrete values, selected from said set of
predefined discrete values.
In an embodiment of the modulator of the current invention said
signal is a time-varying electrical potential.
In an embodiment of the modulator of the current invention said
signal is a time-varying electrical current.
In an embodiment thereof, the discretizing circuit is an inverter
circuit comprising a transistor of the PMOS-type and a transistor
of the NMOS-type.
In a further embodiment thereof, the discretizing circuit
comprising a number of inverters in series, wherein at least one
inverter is provided with a transistor of the PMOS-type and a
transistor of the NMOS-type,
In yet a further embodiment thereof, said number of inverters in
series is an even number.
In an embodiment thereof, each consecutive inverter in said number
of inverters in series comprises transistors designed to operate
with a higher current than the transistors in the previous inverter
in said number of inverters in series.
The invention further relates to a modulator array comprising a
plurality of modulators, for modulating the magnitude of plurality
of beamlets in a multi-beamlet lithography system, each modulator
comprising: at least one means for influencing the direction of a
beamlet; a light sensitive element for receiving light from a
modulated light beam and converting said light into a signal;
discretizing means, coupling to said light sensitive element and to
at least one of means for influencing, for converting said signal
received from said light sensitive element into a discrete signal
having discrete values selected from a set of predefined discrete
values and providing said discrete signal to said means for
influencing.
The invention further relates to a method for transferring a
pattern onto the surface of a target using a lithography system
comprising beam generating means for generating a plurality of
beamlets and modulation means for individually controllably
modulating substantially each beamlet, said method comprising:
retrieving pattern data from data storage means; transforming said
pattern data into at last one modulated light beam; optically
coupling said at least one modulated light beam to said modulation
means comprising an array of modulators with at least one modulator
for each beamlet, each modulator comprising: at least one means for
influencing the direction of a beamlet; a light sensitive element
for receiving light from a modulated light beam and converting said
light into a signal; discretizing means, coupling to said light
sensitive element and to at least one of means for influencing, for
converting said signal received from said light sensitive element
into a discrete signal having discrete values selected from a set
of predefined discrete values and providing said discrete signal to
said means for influencing.
DRAWINGS
The invention will be further elucidated in the following
embodiments of a maskless lithography system according to the
current invention, in which:
FIG. 1a, 1b a an operation scheme of part of the system of the
invention;
FIG. 2a, 2b, 2c free space optical coupling,
FIG. 3a, 3b illumination schemes of a modulation means;
FIG. 4 projection of optical fiber array on modulation array;
FIG. 5a, 5b projection systems for projecting a pattern information
carrying light beam on modulation means;
FIGS. 6a 6d illuminating schemes for the light sensitive
elements;
FIG. 7 coupling of pattern information carrying light beams to
light sensitive elements;
FIG. 8 top-view of FIG. 7;
FIG. 9 optical coupling using optical fiber ribbons;
FIG. 10 modulation means for an electron beam lithography
system,
FIG. 11 free space coupling of pattern information carrying light
beams to modulation means;
FIG. 12 illumination scheme of a modulation means;
FIG. 13 maskless EUV lithography system;
FIGS. 14a 14d illustrate a problem with optical transmission of
signals;
FIG. 15 shows a circuit for a modulator according to the present
invention;
FIG. 16 shows a further embodiment of a circuit for a modulator
according to the present invention;
FIGS. 17a 17b show two further embodiments of circuits for a
modulator according to the present invention;
FIGS. 18a 18e illustrate functional charts of discretizing
means;
FIG. 19 shows an modulator array for a multi-beamlet optical
lithography system;
DETAILED DESCRIPTION OF THE INVENTION
In the modulation means according to the invention, the modulation
means are fed with an optical signal. Substantially each comprising
a light sensitive element, preferably a photodiode. The basic
operation of the modulation means is schematically shown in FIG la.
A pattern information carrying light beam is sent from the control
unit towards the light sensitive element.
If the light sensitive element receives light, a signal is
generated and sent to modulation means. As a result the passing
beamlet will be modulated and not reach the target exposure
surface. If there is no light, there is no signal transferred to
the modulation means. The beamlet passes undisturbed, and finally
reaches the target exposure surface. By moving the target exposure
surface and the rest of the lithography system relative to each
other while sending pattern information towards the modulation
means, a pattern can be written.
It is of course also possible to operate the whole system in the
opposite way as shown in FIG lb. In this case light falling on the
light sensitive element results in the cancellation of the signal
sent towards the modulation means. The passing beamlet will reach
the target exposure surface without any modulation. However, when
the light sensitive element does not receive light, a signal is
sent towards the modulation means, which prevents the passing
beamlet from reaching the target exposure surface.
The attachment of the optical fibers to the modulation means can
give serious complications. In an embodiment, the last part of the
data trajectory uses a different transfer medium. In the latter
case the fibers terminate closely packed thus forming an optical
fiber array. The emitted pattern information carrying light beams
are then sent towards other optical carriers. When the modulation
means are located in a vacuum, it might be preferable to keep the
optical fibers outside the vacuum. In the latter case the emitted
light beams can couple into the lithography system via a
transparent part of the vacuum boundary.
In most cases it is not practical to bring the pattern information
carrying light beams all the way to the light sensitive elements
through optical fibers. In that case other optical carriers can
continue the data transfer. Preferably the optical fibers are
bonded together to form an optical fiber array. The pattern
information carrying light beams then travel towards the light
sensitive elements in a different way. One possible way of data
transfer is to send the light emitted from the fibers towards the
light sensitive elements of the modulation means through the same
environment as wherein the irradiated beamlets are traveling. In
this way free space optical interconnects are created. Another
possible transport medium is an optical wave-guide, which is
located in the structure of the modulation means.
In the case of an optical wave-guide or an optical fiber multiple
wavelengths can be transported through the channels as is commonly
done in telecommunication applications. The space occupied by the
transfer medium then reduces significantly, because several pattern
information carrying light beams share the same channel. The
conversion towards a signal that can be used by the modulators can
be made with an opto-electronic receiver, like a DWDM
multi-wavelength receiver.
The light sensitive element can be any element known in the art
that converts an incoming light signal into any other kind of
signal, like an electric or an acoustic signal. Examples of such
converters are photo cathodes, phototransistors, photo resistances
and photodiodes. In order to meet the high data rate requirements,
the light sensitive element should have a low capacitance, enabling
it to operate at a high frequency. Moreover the element is
preferably easy to integrate in the modulation means. There are
photodiodes that meet the demands mentioned above. The preferred
embodiment uses an MSM-photodiode. The main advantage of this
photodiode is its low capacitance. It is therefore able to operate
at a high frequency. Moreover, the fabrication of a MSM-photodiode
is relatively easy. Another good option would be the use of a
PIN-photodiode. This element also has a low capacitance, but it is
somewhat more difficult to integrate this component in an array.
Another very useful option is an avalanche photodiode.
As already mentioned earlier, the data rate and thus the modulation
frequency are very high. In order to be able to modulate at this
rate suitable switching circuitry is important. Next to the three
optical carriers, which will be discussed below, other related
means to transfer modulated light beams are embodied by the present
invention.
Transfer Options
Free Space Optical Interconnects
When the pattern information carrying light beams are projected on
the corresponding light sensitive elements through the same medium
as wherein the irradiated beamlets are traveling, several
complications arise. It is often not possible to project the
pattern information carrying light beams on the light sensitive
elements perpendicular to the plane wherein the light sensitive
element is located. This can for instance be the case when the
irradiated beamlets are already projected perpendicular to said
plane. Interference between the beamlet and the pattern information
carrying light beam might have an influence on the pattern, which
results in an incorrect data transfer from control unit towards
target exposure surface. To avoid this problem the pattern
information carrying light beams reach the light sensitive surface
of the light sensitive element, say a photodiode, at a certain
angle. However, when this angle of incidence .alpha. increases, the
spot size of the pattern information carrying light beams on the
light sensitive surface of the photodiode increases as well. In
order to address every photodiode individually the spot size of the
pattern information carrying light beams should be less than the
light sensitive surface area of the photodiode. The angle of
incidence .alpha. should therefore be as small as possible.
However, this is not always possible due to obstacles as shown in
FIG. 2a.
Of course with a smart choice of the location of the fiber array 2
and the obstacle 1 most often a problem is avoided. However, this
is not always possible. The present invention includes ways to
reduce the angle of incidence .alpha. without said removal or
replacement of the obstacle 1. First of all, the obstacle 1 can be
made transparent to the pattern information carrying light beams.
If the barrier is for instance an electrostatic lens array, it may
be made of some kind of conductive glass or polymer. Alternatively,
the wavelength of the pattern information carrying light beams can
be chosen in such a way that the obstacle 1 becomes transparent for
these beams. Silicon, for instance, becomes transparent to
wavelengths larger than 1100 nm. So when a wavelength of 1500 nm,
used in standard fiber applications, is used, the emitted beams
will pass a silicon barrier without noticing its existence.
Another possibility to reduce the angle of incidence .alpha.
without removing the obstacle 1 is to use more optical fiber arrays
2. In FIG. 2a a situation is sketched wherein the pattern
information carrying light beams leaving the fiber array 2 are
projected on a plate 3 provided with modulators. The emitted beams
cover the total plate 3. If in this configuration the projected
spot size is too large, the angle of incidence can be reduced by
moving the fiber array 2 away from the modulation means plate 3
perpendicular to the plane wherein the photodiodes are deposited as
is shown in FIG. 2b. As a result the critical angle of incidence
.alpha..sub.1 is reduced. Now the spot size may be limited within
the requirements. However, only half of the plate 3 is illuminated.
By using a second fiber array 2 at the same height at the opposite
side of the modulation plate 3 as shown in FIG. 2c, the entire
plate 3 is illuminated and the spot size is small enough. Both
optical fiber arrays 2 comprise half the amount of fibers compared
to the original one. By selecting the right amount of optical fiber
arrays 2, a plate provided with an array of light sensitive
elements can be illuminated with the desired angle of incidence
.alpha..sub.1.
FIGS. 3a and 3b show a top view of a squared and a rectangular
modulation plate 3. The dotted lines bound the area illuminated by
one fiber array. As already explained earlier, one fiber array may
not be enough. In that case for instance 2,4 or 6 optical fiber
arrays 2 can be used to illuminate the entire plate within the
requirements.
Furthermore it is possible to couple the pattern information
carrying light beams into the system via some reflections. The
obstacle 1 can for instance be coated with a reflective material.
Moreover additional mirrors can be placed on strategic positions in
the system to create the desired angle of incidence.
The pattern information carrying light beam has a diameter of about
50 150 .mu.m when it leaves the optical fiber residing in the
optical fiber array in the case that a multimode optical fiber is
used. A single mode fiber only has a diameter of about 1 10 .mu.m.
The light sensitive surface of a photodiode can be in the order of
10 30 microns squared. In an embodiment, multimode optical fibers
are used, so the diameter of the pattern information carrying light
beams leaving the optical fiber array need to be reduced.
Furthermore some kind of focusing has to be arranged to realize a
projection with the correct resolution.
An optical assembly may be needed to perform both reduction and in
focus projection of the pattern information carrying light beams.
There are two properties of the fiber array that can be reduced.
First the diameter of the light beams leaving the optical fiber
array 2 can be demagnified. Secondly the distance between two
adjacent light beams, the so-called pitch, can be reduced by
optical means. The in focus projection of the light beams leaving
the optical fiber array 2 on the modulation plate 3 can most easily
be achieved when both optical fiber array 2 and modulation array 3
are lying parallel to each other. If the two planes are not
parallel the spot size of each individual light beam on the
modulation array 3 will vary. The projection of the fiber array 2
on the modulation plate 3 is done with a lens 5. Often the light
beams are projected on the modulation plate 3 with an angle of
incidence unequal to zero. The optical fibers 4 in the optical
fiber array 2 may then be arranged in such a way that the light
beam leaving the optical fiber is directed towards the lens as is
shown in FIG. 4. In this way a sufficient illumination of the lens
5 is ensured.
When the lens 5 is located exactly in the middle between the
optical fiber array 2 and modulation plate 3, 1:1 projection takes
place. Moving the lens towards the modulation plate 3 reduces both
diameter and pitch of the pattern information carrying light beams.
Moving the lens 5 in the other direction, i.e. in the direction of
the optical fiber array 2, will result in an increase of both
parameters.
For an optimum performance regarding both reduction and projection
more lenses may be needed. A possible configuration with two lenses
6 and 7 is shown in FIG. 5a. The entire image and thereby the
diameter of every individual pattern information carrying light
beam 5 leaving the optical fiber array 2 is reduced. In an
embodiment with obstacles, mirrors can be used to project the light
beams on the light sensitive elements.
In some cases the beam diameter needs to be reduced more than the
pitch between the adjacent light beams 8. A micro lens array 9
positioned between the optical fiber array 2 and a projection lens
7 can arrange this. Each individual lens corresponds to a single
fiber 4 in the optical fiber array 2. The diameter of each pattern
information carrying light beam 8 leaving the optical fiber array 2
is individually demagnified in this configuration as depicted in
FIG. 5b. A projection lens 7 focuses all demagnified beams onto the
corresponding light sensitive elements. When direct projection is
impossible due to some obstacle, mirrors can be used to project the
pattern information carrying light beams on the light sensitive
elements at the desired angle of incidence .alpha..
Another potential problem related to the spot size is cross talk
between adjacent pattern information carrying light beams emitted
from the fiber array 2. Applying several measures can reduce this
cross talk. Consider again that the beams are projected on an array
of modulation means wherein the light sensitive surfaces of for
instance photodiodes are all lying within one plane at one side of
the array.
A first solution to the cross talk problem is depicted in FIG. 6a.
The area between adjacent light sensitive elements is covered with
a reflective layer 10. The major part of the incoming light beam
falls on the light sensitive converter element 11. The part of the
light beam that is not falling on the element 11 is reflected back
into the system, without affecting any of the adjacent elements.
Coating the light sensitive elements 11 with an anti-reflective
layer can enhance the light detection efficiency even further.
A second measure to reduce the cross talk is the use of a diffusive
layer 12 on top of the entire array 3 as shown in FIG. 6b. The
incoming light is now scattered in all directions. Due to the
scattering the light intensity of the reflected beam drops
drastically. A disadvantage of this approach is that it demands a
higher threshold level of the light sensitive element 11 to
compensate for the increase of the background radiation.
A third way to reduce the cross talk is the use of a filter located
on top of the light sensitive converter element 11. Examples are a
wavelength filter 13 as shown in FIG. 6c, or a polarization filter.
The wavelength filter 13 enhances the selectivity for a certain
wavelength. As a result waves coming from adjacent patterned beams
with a slightly different wavelength are filtered out. A filter,
which only transmits light polarized in a predetermined direction
works, can have the same effect.
Yet another possible measure is to make the light sensitive
elements 11 only sensitive for light coming from a predetermined
direction. This can be done by incorporating small prisms 14 or
gratings 15 in the modulation array 3 as depicted in FIG. 6d. Only
the light that falls on the light sensitive element 11 at the
correct angle and coming from the right direction is used in the
modulation process. Light coming from all other directions is
excluded.
Optical Wave-guides
A second possibility to transfer the pattern information carrying
light beams leaving the optical fiber array 2 towards the light
sensitive elements 11 embedded in the modulation means is the use
of planar optical wave-guides. Planar optical wave-guides can be
thought of as optical fibers embedded into or onto a substrate.
Consider again the array of modulation means 3. Planar optical
wave-guides may be integrated in this array. Then a system as
schematically shown in FIG. 7 can be constructed. Each individual
pattern information carrying light beam 8 leaving the optical fiber
array 2 has to be coupled into the corresponding optical wave-guide
16. This can be done directly or via an array of lenses 17 as shown
in FIG. 7. Each lens then couples an individual pattern information
carrying light beam 5 into the entrance point of the corresponding
planar optical wave-guide 16. The optical wave-guide 16 transports
the pattern information carrying light beam 8 through the
modulation array 3 towards the correct light sensitive element 11.
The light sensitive element 11 converts the pattern information
carrying light beam 8 into a sequence of signals, which activate or
deactivate the modulators 18. Consequently the incoming beamlet
will be controlled according to the pattern information. The
sequence of signals in this embodiment is transported through
electrical wires 19 embedded in the modulation array 3 towards the
modulators 18.
FIG. 8 shows a top view of the same configuration as depicted in
FIG. 7. In this case two fiber arrays 2 are used to control all the
modulators 18. However, any number of arrays 2 is applicable. The
light sensitive elements 11 are represented by squares, while the
modulators 18 are represented by circles. Only two trajectories of
pattern information carrying light beams 8 are shown for
clarity.
Optical Fibers
The third possibility for the data transfer from the control unit
towards light sensitive element 11 is to use optical fibers for the
entire trajectory. The major problem with this approach is the
connection of the individual fibers 4 to the structure wherein the
modulation means are integrated. Again imagine that a modulation
array 3 is used. Connecting the individual fibers 4 to this array 3
may give problems when for instance this array 3 is moving for
scanning purposes. Mechanisms like stress and friction are
introduced in the region of attachment. Eventually the connection
can break. A possible measure to avoid this problem is to combine a
group of optical fibers 4 to form a fiber ribbon 20. The ribbon 20
is then connected to at the side of the modulation array 3 as shown
in FIG. 9. In the figure two ribbons 20 are shown. Another number
of ribbons 20 is also possible. Two exemplary trajectories of
optical fibers within the fiber ribbon are schematically shown with
dashed lines. The light sensitive elements 11, represented in the
figure as squares, may be located close to the contact of the fiber
ribbon 20 with the modulation array 3, but they may also be located
closer to the incoming beamlets. Preferably the optical signals are
converted in electrical signals. The electrical signals are
transferred through on-chip electrical wires 19 towards the
modulators 18, represented by circles, located in close proximity
of the corresponding incoming irradiated beamlets.
EXAMPLES
The next two sections describe two examples of maskless lithography
systems embodied by the present invention.
Example 1
Maskless Electron Beam Lithography System
FIGS. 10 12 show several parts of this system. In the maskless
electron beam lithography system used in this example, the system
comprises an aperture plate comprising electrostatic deflectors 21
to deflect incoming electron beamlets 22 passing through the
apertures 23. This plate will be referred to as the beamlet blanker
array 24. When the electron beamlets 22 have passed the beamlet
blanker array 24 they will reach a second aperture array on which
their trajectory will terminate when they are deflected. The second
plate is referred to as the beamlet stop array 25.
The modulation concept of this lithography system is shown in FIG.
10. Incoming electron beamlets 22 are projected on the beamlet
blanker array 24. The positions of the electron beamlets 22
correspond to the positions of the apertures 23 in the plate 24.
The beamlet blanker plate 24 comprises a deflector element as
modulation means. In this example said deflector element comprises
an electrostatic deflector 21. Depending on the received
information the deflector 21 located in the beam blanker array 24
will be turned on or off. When the deflector 21 is turned on, an
electric field is established across the aperture 23, which results
in a deflection of the beamlet 22 passing this aperture 23. The
deflected electron beamlet 27 will then be stopped by the beamlet
stop array 25. In this case no information will reach the target
exposure surface. When the deflector 21 is turned off the beamlet
will be transmitted. Each transmitted beamlet 28 will be focused on
the target exposure surface. By moving the target exposure surface
and the assembly of arrays relatively to one another and by
scanning the beamlets with for instance an additional beamlet
deflector array a pattern can be written.
FIG. 11 shows a possible configuration of the usage of free space
interconnects in this maskless lithography system. The pattern
information carrying light beams 8 coming out of the data storage
medium in the control unit leaving the optical fiber array 2 are
demagnified by two lenses 29 Alternatively also other
configurations as for instance shown in FIG. 5 can be used. The
pattern information carrying light beams 8 are then projected on
the beamlet blanker plate 24 with a mirror 30 and a focusing lens
7. The angle of incidence .alpha. ranges from 0 and 80 degrees. If
_ is larger than 80 degrees or a smaller angle is desired due to
other complications the beamlet blanker plate 24 can be illuminated
with more than one fiber array 2 as is shown in FIG. 12. In the
depicted situation 4 fiber arrays 2 are illuminating the beamlet
blanker plate 24. In FIG. 12 the 4 corresponding focusing lenses 7
are depicted, focusing the pattern information carrying light beams
8 on the respective part of the beamlet blanker plate 24.
Example 2
Maskless EUV-Light Optical Lithography System
The maskless EUV system in this example, schematically shown in
FIG. 13, comprises a spatial light modulator (SLM). Maskless
lithography systems using an SLM can be found in for instance
WO0118606. The SLM comprises an array of mirrors, which reflect the
incoming light beams in such a way that the beam eventually is
blanked or transmitted. An example of such an SLM is a deformable
mirror device (DMD). A DMD is controlled in the same way as the
electrostatic deflector array shown in the first example. The
modulation signals couple into the system from the back or from the
side. The most probable configuration would be a backside control
of the modulation. By providing the backside of each mirror with a
light sensitive element, the control can be done with the use of
the same optical carriers as mentioned before. Probably the use of
free space optical interconnects is the most convenient option.
A schematic drawing of the operation is shown in FIG. 13. A laser
emits a light beam, which is split into a plurality of beamlets by
a beam splitter. The plurality of beamlets is projected on the SLM.
The pattern information carrying light beams sent from the control
unit towards the SLM control the transmission probability of
beamlets coming from the beam splitter. The transmitted beamlets
are focused on the target exposure surface with a lens. By moving
the target exposure surface and the rest of the system relatively
to each other a pattern can be written.
The operation of the modulation means is furthermore influenced by
characteristics of the data converter in the control unit, which
emits the at least one pattern data carrying light beam. The light
falling on the light sensitive element generates a photon-induced
signal, S.sub.lse. In most light sensitive elements a higher
intensity of the incoming light, I.sub.light, generates a more
intense signal as is shown in FIG. 14a. The created signal can
either be a modulated current or a modulated potential, which value
follows the intensity changes of the light emitted by the data
converter in the control unit.
Consider the ideal operation of the modulation means with two
modulation options: the beamlets are either modulated or not.
Following the mode of operation depicted in FIG. 14a, the output
signal of the light sensitive element switches from an "off"-state
to an "on"-state upon the reception of a light beam as is depicted
in FIG. 14b. It remains in this situation until the control signal
turns "off" again, i.e. until no light is detected by the light
sensitive element. However, the pattern data carrying light beam
emitted by said data converter in the control unit has unintended
intensity variations. Furthermore the intensity of the light when
the control unit sends an "off"-signal is not always zero. The
results of these variations and offset are shown in FIG. 14c. The
variations and offset in the light intensity cause similar
variations in the output signal generated by the light sensitive
element. The beamlets modulated with this signal experience
unintended displacements and aberrations. Consequently patterning a
surface of a target with high-resolution features becomes extremely
difficult, if not impossible.
In an embodiment of the present invention, the aforementioned
performance limitations are avoided by providing each modulator
with discretizing means. The continuous output signal of the light
sensitive element serves as an input signal to this discretizing
means. The output signal of the discretizing means, which is the
signal responsible for the actual modulation of the beamlets, is a
continuous signal with a set of discrete, allowable and
predetermined values. In the easiest case, the binary case, the
output signal resulting from the discretizing means can have two
values: one value corresponding with "modulation on", the other
value corresponding with "modulation off".
The discretizing means preferably comprises a comparator circuit of
some sort. When the output signal is a potential, a potential
comparator circuit of some form can be used to create a discrete
set of possible values of the output modulation potential. In the
case that the light sensitive element is used as a current source,
a current comparator circuit of some form can be used to discretize
the signal and provide a modulation current which can have a
limited set of allowable, discrete values. Additionally, the
discretizing means can be provided with an IV or VI-converter. By
adding the converter it becomes possible to compare a current and a
voltage.
A comparator generally has two input signals and one output signal.
Often, one input is a constant reference signal S.sub.ref, and the
other input signal is a time-varying signal. The time-varying
signal used in the present invention is the output signal of the
light sensitive element, S.sub.lse. The comparator compares the
input value of S.sub.lse with the reference value of S.sub.ref.
When the difference stays below a certain threshold value, S.sub.t,
the output signal is in a first state, wherein the value of this
signal is fixed at a fixed first value. When the difference exceeds
the threshold value due to for instance the reception of an optical
control signal by the light sensitive element, the output signal
switches to a second state. In this state the signal has a second
fixed value, different from said first fixed value. A further
increase of the difference does not change the value of the output
signal anymore. Only a decrease of the difference below the
threshold value, for instance caused by the removal of the optical
signal falling on the light sensitive element, resets the
modulation potential back to its first state. In the present
invention the discretized output signal of the comparator is used
as modulation signal S.sub.mod.
The consequences of the addition of the discretizing means on the
modulation signal in the binary case are shown in FIG. 14d. The
modulation signal switches at t.sub.l between said first fixed
value to said second fixed value because at that time the
difference between the values of S.sub.lse and S.sub.ref exceeds
said threshold value S.sub.t. The threshold value can be tuned by
accurate selection of suitable components.
Consider the example of the beam blanker array (BBA). FIG. 15 shows
an inverter circuit that is used to charge and discharge the
deflector electrodes upon the reception of corresponding optical
control signals. The circuit is able to switch between two
predetermined well-defined voltages, a high potential V.sub.h and a
low potential V.sub.l. The circuit comprises a PMOS-transistor
(upper transistor) 100 and an NMOS-transistor (lower transistor)
101. The base of both transistors 100 and 101 are connected to each
other and to the lightsensitive element 102. The collectors of both
transistors are also connected to each other. The emitter of
transistor 100 is connected to high potential V.sub.h and the
emitter of transistor 101 to low potential V.sub.l. The collector
of both transistors 100 and 101 are also via capacitor 103
connected to low potential V.sub.l. The circuit of FIG. 14 operates
in the following way following the first mode of operation.
When light is detected by the light sensitive element a
photon-induced current I.sub.lse is created. In this circuit the
light sensitive element 102 thus serves as a light sensitive
current source. The capacitor 103 between the gate and the source
serves as an IV-converter, since it behaves as a resistor at high
frequencies. So the current charges the capacitors of the
transistors 100, 101 such that:
V.sub.GS,PMOS-V.sub.H>V.sub.T,PMOS and
V.sub.GS,NMOS-V.sub.L<V.sub.T,NMOS
wherein V.sub.GS is the gate-source voltage and V.sub.T the
threshold voltage of the PMOS-transistor 100 and the
NMOS-transistor 101 respectively. As a result the PMOS-transistor
100 now switches "on" and the NMOS-transistor 101 switches "off".
The established current between drain and source of the
PMOS-transistor 100 charges the deflection electrode to a potential
equal to V.sub.H. Consequently the charged particle beam passing
through a corresponding aperture is deflected.
When no light is incident on the light sensitive element, the
conditions change such that: V.sub.GS,PMOS-V.sub.H<V.sub.T,PMOS
and V.sub.GS,NMOS-V.sub.L>V.sub.T,NMOS
Thus the NMOS-transistor 101 switches "on" and the PMOS-transistor
100 is switched "off". Consequently the deflector electrode
discharges and the initial situation is restored. Note that in this
configuration shown two threshold values are used.
It is also possible to use more threshold values by putting
additional inverter circuits as shown in FIG. 15 in parallel.
Eventually a discrete set of modulation potentials can be created
thus enabling controlled modulation of the beam intensity of each
individual passing charged particle beamlet. By selecting
appropriate transistors an optimized setup can be designed for each
individual case. The use of more than one threshold values is not
limited to this example. In fact the use of more than one threshold
value can be useful in any maskless lithography system, especially
in lithography systems embodying "gray writing" i.e. a patterning
method using a discrete set of dose values.
The circuitry depicted in FIG. 15 works best at a high frequency
when the capacitance of the used transistors is of the same order
of magnitude as the sum of the parasitic capacitance and the
capacitance between the electrodes of the electrostatic deflector.
When the capacitance of the transistors is much smaller than the
modulation capacitance, the applied current needs to be amplified.
The amplification of the applied current can for example be
established by putting several inverters (transistors 100 and 101))
in series as schematically shown in FIG. 16. The transistors
(100-1, 100-2, 100-3, 101-1, 101-2, 101-3) in each consecutive
stage (i.e. stage with 100-1 and 101-1, stage with 100-2 and 101-2,
stage with 100-3 and 101-3) are designed to operate with a higher
current than the stage before.
The performance of the circuits depicted in FIGS. 15 and 16 operate
less successful at lower frequencies. At these lower frequencies,
the input potential of the inverter circuit can become undefined,
and it may take too much time to reset the value in time for a next
cycle of operation. Under these conditions a feedback mechanism can
improve this. FIG. 17a shows a circuit comprising a feedback
arrangement with a PMOS transistor 104. In this circuit, capacitor
103 is not connected to both collectors, but to the base of
transistor 104. The emitter of transistor 104 is connected to high
potential V.sub.h and its collector to light sensitive element
102.
The feedback arrangement resets the input signal of the inverter
circuit and keeps it at this value until light is falling on the
light sensitive element again. Note that the specific configuration
shown only operates successfully in the second mode of operation,
schematically shown in FIG. 14b i.e. light signal on means
electrostatic deflection off. Again the current can be amplified
using an arrangement similar to the one shown in FIG. 16. However,
an even number of inverters is preferred, as arrangements with an
odd number of inverters operate less successfull at a
low-frequencies.
A leakage current through the feedback transistor 104 has a
negative influence on the performance of the circuit. To avoid said
influence a second complementary transistor 105 can be positioned
in parallel with the feedback transistor 104 as is depicted in FIG.
17b. The leakage current through the feedback transistor 104 is now
entirely compensated by the leakage current through said
complementary transistor 105 which is connected with its collector
to the light sensitive element and with its base and its emitter to
high potential V.sub.h. Effectively no leakage current is
experienced.
FIG. 18 illustrates the functions of the discretizing means. The
easiest embodiment of said discretizing means only comprises a
comparator as is depicted in FIG. 18a. The comparator compares two
input values, and depending on the result of said comparison, an
output value is assigned. In the present invention the input value
is the value of the signal created by the light sensitive element
S.sub.lse. The second input value I is a certain predefined
threshold value. The output value is chosen from a set of
predetermined values and depends on the result of the
comparison.
Said output value is then assigned to the modulation signal
S.sub.mod, which modulates the beamlet responsible for patterning
the surface to be exposed. In numerous cases the range of the
signal generated by the light sensitive element is not aligned
properly with the threshold value. This problem can be solved by a
adding at least one differentiator to the discretizing means as is
shown in FIG. 18b. Before reaching the comparator the input coming
from the light sensitive element S.sub.lse, is properly adjusted by
subtracting a reference signal S.sub.ref from S.sub.lse. The
difference, S.sub.lse--S.sub.ref, is then compared with the
threshold value by the comparator. The embodiments shown in FIGS.
15 17 all operate with the mechanism shown in FIG. 18b as depicted
in FIG. 18c. The reference signals correspond with the signals at a
constant value, V.sub.H and V.sub.L and are connected to the
source-side of the transistors. The threshold value, V.sub.T, is an
intrinsic value of each transistor. The signal S.sub.lse is applied
to the gate of the transistor. The comparator now compares the
gate-source difference and the threshold value in each transistor
and decides whether a current I.sub.DS can run or not. A suitable
configuration of these elements then results in the desired
discretizing operation.
Another embodiment of the discretizing means is shown in FIGS. 18d
end 18e. In FIG. 18d, the signal from the light sensitive element
is directed directly to a comparator, and via a delay and,
optionally, a multiplier, to the comparator.
This circuitry is also called constant fraction discriminator. In
this way, not a constant, predefined threshold is used. The
threshold is in fact based upon the difference between the value of
the current signal and the value of the signal a predefined amount
of time ago (depending on the delay). A variation on this
embodiment is shown in FIG. 18e.
Not only a maskless charged particle lithography system but also a
maskless optical lithography system can be provided with
discretizing means. An example of a modulator array in the latter
system is shown in FIG. 19. In the shown configuration optical
beamlets 107 are projected on an array of micro-mirrors 109. Upon
recieving (or interruption) of a control signal the direction of
reflection changes.
According to the current invention, the control signals are sent
optically, in the shown embodiment through fibers 106 and free
space, towards corresponding light sensitive elements 108. Again
intensity differences in these optical control signals lead to
different intensities of the resulting electrical control signals
created by said light sensitive elements 108.
In FIG. 19, the second control line from the left transfers a
signal with a large intensity N, larger than the intensity n of the
fourth control line, which only carries a signal with an intensity
n. This is due to the smaller intensity of the optical control
signal falling on the light sensitive element 108 of the latter
control line. If these signals would be directly applied to the
corresponding micro-mirrors 109, the corresponding beamlets 110
exposing the target surface would be reflected at different angles.
To eliminate this difference, discretizing means 111 are positioned
between the light sensitive elements 108 and their corresponding
micro-mirrors 109. As a result, mirrors 109 deflect the beamlets
110 that expose the target surface in an identical way upon
receiving an optical control signal, independent of the intensity
of the optical control signal 107.
In this particular embodiment shown in FIG. 19, the reception of an
optical control signal results in a reflection of the exposure
beamlets 110 in direction close to the source and away from the
target. This state is denoted in FIG. 18 as "0". Without the
reception of the signal the exposure beamlets will reach the
target. The corresponding state is denoted in FIG. 19 as "1".
It is to be understood that the above description is included to
illustrate the operation of the preferred embodiments and is not
meant to limit the scope of the invention. From the above
discussion, many variations will be apparent to one skilled in the
art that would yet be encompassed by the spirit and scope of the
present invention.
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